1. Field of the Invention
The present invention relates to a means for reducing display unevenness, and more specifically, the present invention relates to a display device given a reduction measure of display unevenness caused by dispersion of energy density of a laser beam, and a manufacturing method thereof.
2. Description of the Related Art
A thin film transistor (hereinafter referred to as a TFT) is used as an element for driving a liquid crystal display device, EL(Electro Luminescence)display device and the like. A glass substrate has been utilized for the purpose of manufacturing TFTs at low cost. It is difficult to manufacture a TFT in a way that requires a long time heat treatment at approximately equal to or more than 600° C. Thus, a technique for manufacturing a TFT in a low temperature process of at most 600° C. for highest temperature in the process has been developed. A crystallization method using a laser beam is generally utilized for a method for manufacturing a crystalline semiconductor film by such a low temperature process.
When dispersion of energy density of a laser beam due to instability of output in a laser oscillator is caused in a method for manufacturing a crystalline semiconductor film using a laser beam, hereby, film quality of the crystalline semiconductor film is also varied. It is know that dispersion of a TFT electrical characteristic is generated by the crystalline dispersion of the crystalline semiconductor film. Specifically, when an electrical characteristic of a TFT for driving a pixel is varied, brightness unevenness or display unevenness such as gradation unevenness is caused in a display image.
Accordingly, two laser oscillators are used alternatively, for example, maintenance is performed to the one oscillator while performing crystallization using one oscillator so as to obtain stable output from the laser oscillators all the time, then, an attempt to reduce film quality dispersion of crystalline semiconductor film due to dispersion of energy density of a laser beam has been performed. (For example, U.S. Pat. No. 3,135,643)
In the case of using the above-described method, there is an effect that the process is not required to be interrupted for the maintenance. However, the frequency of maintenance itself can not be reduced. Thus, the development for a method in which trouble involved in maintenance or the like is reduced, energy dispersion of a laser beam is reduced with more convenient method, and more satisfactory display image is obtained is required.